The distributed feedback semiconductor laser generates stimulated emission of light by distributed optical feedback to an active layer with a diffraction grating provided near the active layer. The device can produce stimulated emission of excellent lasing spectral characteristics by a relatively simple construction, Therefore, it has been the target of various R & D efforts and is expected to be used as a light source suitable for a long-distance and large-capacity optical communication system, an optical information processing system, an optical memory system and an optical measuring instrument.
Such distributed feedback semiconductor laser has an optical waveguide structure, wherein an active layer is surrounded with transparent hetero-junction semiconductor layers, for efficient stimulated emission. R & D efforts are recently directed toward distributed feedback of light by periodically changing the refractive index in a transparent optical waveguide layer which is placed very close to the active layer. In that case, a diffraction grating, having triangular cross section for instance, is formed on the interface of the optical waveguide layer on the side farther from the active layer.
In the light distributed feedback by such index coupling, however, feedback to phase cannot be matched for the light in Bragg wavelength which is reflected correspondingly to the period of the thickness change in the optical waveguide layer. Because of this phase match condition, stable lasing cannot be obtained and two longitudinal lasing mode whose wavelengths are separated symmetrically in the vertical direction across the Bragg wavelength may possibly be generated at once. Even if only one such longitudinal-mode-lasing takes place, it is difficult to select previously which of the two wavelengths would be lasing. So, the precision in setting lasing wavelength is seriously deteriorated.
In sum, the light distributed feedback using index coupling which is based on the periodical perturbation of the refractive index in the optical waveguide layer has an inherent problem of degeneracy by longitudinal lasing mode of two wavelengths, which is difficult to avoid.
There have been proposed various solutions for the problem. One of them proposed a structure to shift the phase by 1/4 wavelength substantially at the center of the diffraction grating. Those proposals, however, are not quite effective as they tend to make the construction of a laser more complicated, require additional manufacturing steps only for the solution of the degeneracy and need formation of anti-reflection coating on the facets of the laser.
Kogelnik et al. proposed a theory in their paper titled "Coupled-Wave Theory of Distributed Feedback Lasers", Journal of Applied Physics, 1972, Vol. 43, pp. 2327-2335, whereas a stop band is produced around the Bragg frequency when distributed feedback of the light is conducted by index coupling, if distributed feedback is conducted by gain coupling based on the periodical perturbation of gain factors, such stop band would not be produced and longitudinal mode lasing of exclusively single wavelength would be obtained. They did not mention in their paper the specific construction to realize the theory, rather they merely discussed on the result of their theoretical studies.
Some of the present inventors have invented novel semiconductor lasers applying the basic theory of Kogelnik et al., and filed patent applications as follows:
Japanese Patent Application No. 63-189593, filed on Jul. 30, 1988 PA1 Japanese Patent Application No. 1-168729, filed on Jun. 30, 1989 (Publication No. JP-A 3-34489) PA1 Japanese Patent Application No. 1-185001 to 1-185005 filed on Jul. 18, 1989) (Publication No. JP-A 3-49283 to 3-49287) PA1 High (active layer)-intermediate-high-intermediate . . . PA1 Low-intermediate-low-intermediate . . . PA1 (1) The periodic changes in refractive index caused by the absorptive layer and the intermediate refractive index layer are canceled by the periodic changes in refractive index caused by the lower-refractive-index layer and the intermediate-refractive-index layer. PA1 (2) The periodic changes in refractive index caused by the absorptive layer and the lower-refractive-index layer are canceled by the periodic changes in refractive index caused by the lower-refractive-index layer and the intermediate-refractive-index layer. PA1 (3) The periodic changes in refractive index caused by the absorptive layer and the intermediate-refractive-index layer are canceled by the periodic changes in refractive index caused by the intermediate-refractive-index layer and the lower-refractive-index layer. PA1 (4) The periodic changes in refractive index caused by the absorptive layer and the lower-refractive-index-layer are canceled by the periodic changes in refractive index caused by the intermediate-refractive-index layer and the lower-refractive-index layer. PA1 k.sub.0 : wave number in a free space PA1 .beta..sub.0 : propagation constant in z direction PA1 A.sub.q : component of the q-th order when the squares of the refractive indices is Fourier expanded in the z direction PA1 .epsilon..sub.0 : electric field intensity PA1 P: constant obtained by integrating {.epsilon..sub.0 (x) .epsilon..sub.0 * (x)} in the x direction
The inventors succeeded in realizing distributed feedback by gain coupling by the invention constructions which were indicated in the respective specifications and drawings. Many of the constructions shown in these patent applications are provided with periodical corrugation on the surface of an active layer to utilize the periodical perturbation of gain factors caused by the changes in thickness.
The refractive index of the active layer is usually different from that of the surrounding layers as it is necessary to confine the light. If the active layer is corrugated, the refractive index inevitably changes periodically. In other words, the construction having a corrugated surface on the active layer did not achieve the distributed feedback by gain coupling only, but was still subject to the effect of perturbation caused by the index coupling.
Some of the present inventors therefore proposed a design that would diminish the perturbation caused by the index coupling so as to obtain the perturbation caused by the gain coupling alone in the paper, "Purely gain-coupled distributed feedback semiconductor lasers", by Y. Luo, Y. Nakano, K. Tada, T. Inoue, H. Hosomatsu and H. Iwaoka, Appl. Phys. Lett. 56 (17), Apr. 23, 1990, pp. 1620-1622. According to the proposed construction, the thickness of an active layer is periodically changed to provide gain coupling components, and the perturbation of the refractive index due to the corrugated surface of the active layer is canceled by the refractive index perturbation of another corrugation provided nearby with the opposite phase. The GC-DFB-LD which does not substantially contain the index coupling components is herein referred to as a "pure GC-DFB-LD".
Because the overflow of carriers from the active layer should be inhibited, materials of the layers on both sides of the active layer should have sufficiently wide band gap compared to that of the active layer. Such materials, however, have low refractive index, and therefore tend to change the magnitude of the perturbation of refractive index sensitivity to the forms of the two corrugations, for instance, the tooth height of the two corrugations. In order to effectively cancel the perturbation of refractive index, an extremely high manufacturing precision is required.
In the GC-DFB-LD of GaAs based materials which has been realized so far, it was not considered to be a problem because high reproducibility both in their diffraction gratings and the growth shape was relatively easily realized with AlGaAs. But when index coupling components are attempted to be completely eliminated in a device of GaAs based materials, or when the material is inferior in manufacturing precision for canceling the perturbation of refractive index, the process should be controlled thoroughly and strictly.
It is an object of this invention to provide a distributed feedback semiconductor laser which can solve the above mentioned problems of the prior art, which can diminish distributed feedback caused by index coupling and which can obtain the distributed feedback caused mainly by gain coupling.